What time period does mamma mia take place in?

I don't understand.. okay.. the story says that donna and her lovers hooked up in the "flower power" this suggests the 60's early 70's at the latest. Sophie says she's 20.. if it's set in the present, then she would be mid-30's. The original musical premiered in 99' so that still wouldn't work. Anyone have a solution for this plot hole?